It has been demonstrated in industrial case studies and the technical literature that the assumption of a fatigue limit is not valid for steel welds, with fatigue failures occurring in the gigacycle (1 billion stress cycle) regime. Structures and components across a range of industries have service lives above 107 cycles, but there is limited fatigue data in this domain. Conducting fatigue tests in the gigacycle regime is only feasible by using ultrasonic fatigue testing machines in which specimens are excited at 20 kHz.
This research focuses on butt joints of S275 structural steel, fabricated using gas-shielded flux-core arc welding. A novel specimen was produced which captures the geometric stress concentration at the weld toe, as an alternative to typically used cylindrical hourglass shaped specimens. Specimens were tested in fully reversed axial loading at room temperature using a Shimadzu USF-2000A commercial ultrasonic fatigue testing machine, up to the gigacycle regime. An elevated fatigue strength is often observed with ultrasonic fatigue testing for materials with a ferritic microstructure, a phenomenon known as the frequency effect. Therefore, specimens with the same gauge geometry were tested at 20 Hz using a conventional servo-hydraulic machine. This study presents the fatigue data obtained at both ultrasonic and conventional loading frequencies alongside complementary metallography, microhardness and fractography results.